Such electric motors have been known for a long time from the prior art and they are used in very different fields and with very different dimensions. By way of example, electric motors are used to drive streetcars or subways, but also to drive e.g. an artificial heart or a prosthesis.
Here, the functional principle is always the same. In one embodiment of such an electric motor, the stator, i.e. the stationary component, has at least three segments which each have an electromagnet. A current can be applied to these electromagnets separately from one another such that different magnetic fields with different orientations and strengths can be generated in the three segments. Here, the stator has, for example, a round embodiment with a central opening. Situated therein is the rotor, which is present e.g. in the form of a permanent magnet. If an electric current is now applied to some of the electromagnetic elements, the magnetic field is generated, said magnetic field interacting with a magnetic field of the rotor and transferring a torque to the rotor such that the latter starts to rotate. Here, the magnitude and the direction of the torque depend on the directions of the magnetic field lines in the mutually interacting magnetic fields. What can be achieved by skillful switching of the currents which flow through the various electromagnets of the stator is that a torque always acts on the rotor in one direction such that the rotor of the electric motor is kept in motion. Naturally, it is also possible to arrange the rotor outside of the stator or to provide the rotor with electromagnets. It is also possible to embody rotor and stator in the form of electromagnets.
In order to be able to operate such an electric motor in the most ideal manner, it is necessary to ensure that the torque applied to the rotor by the interaction of the various magnetic fields is as large as possible. In order to be able to ensure this, it is necessary to know the position of the rotor relative to the stator in order to know and to be able to exploit the angle relationships between the interacting magnetic fields in the best possible way.
Therefore, DE 196 45 998 A1, for example, has disclosed an artificial heart arrangement with an electric motor, in which, when the electric motor stands still, the position of the rotor relative to the stator can be established. To this end, an electric current is applied to the different segments and the electromagnetic elements situated therein and the inductances are subsequently measured. From this, it is possible to draw conclusions about the orientation of the rotor relative to the stator.
If the electric motor is operated with a constant rotational speed or rate of rotation, knowledge about this rate of rotation and the one-time determination of the position of the rotor relative to the stator are sufficient to ensure an ideal operation of the electric motor. However, particularly in the case where the rotational speed or the rate of rotation are not constant, for example because the electric motor is currently being started up or the rate of rotation depends on the load, an ideal operation of the electric motor is not possible from a one-time determination of the position of the rotor relative to the stator. In this case, the respective position of the rotor needs to be determined during operation as well.
The prior art has disclosed electric motors which achieve this by way of special sensors, e.g. Hall sensors. A disadvantage thereof is that the number of components for such an electric motor greatly increases as a result thereof and, moreover, each one of the sensors requires dedicated cabling and a dedicated power supply such that the production outlay for such electric motors greatly increases.
The prior art has disclosed sensor-less electric motors, in which the position of the rotor can be determined without additional sensors. Here, the electric motor is operated in such a way that at least one of the segments is always operated in a de-energized manner. This means that current only flows through the respective other electromagnets and so only these can build up a magnetic field which transfers a torque to the rotor. The electromagnetic element in the respective third segment is used as a sensor or measurement instrument, with a measurement relating to how large an induced electric voltage is being carried out in this element. Here, the segment operated in a de-energized manner is the segment in which this induced electric variable has a zero crossing. By determining this zero crossing, it is possible, at least theoretically, to determine the position and the time at which this position was assumed. Disadvantages of this method include, firstly, that at least one of the segments cannot transfer a torque onto the rotor in each case and, secondly, that the determination of a zero crossing can be afflicted by significant measurement errors, particularly in the case of a slow rate of rotation.
DE 10 2008 059 052 A1 and DE 198 46 831 A1 have disclosed methods by means of which the rotor position of an electric motor is determinable without a separate sensor.